This invention relates to oxygen sensors and, more particularly, to solid-state oxygen sensors.
An electrochemical zirconia solid electrolyte oxygen sensor has been used for monitoring oxygen concentration in various applications, such as automobiles, to monitor exhaust gas composition and control air-to-fuel ratio for reducing harmful emission components and improving fuel economy. Solid-state electrochemical zirconia oxygen sensors are divided into two groups: (1) potentiometric or logarithmic air/fuel sensors; and (2) amperometric or linear air/fuel sensors. Potentiometric sensors are ideally suited to monitor the air-to-fuel ratio close to the complete combustion stoichiometry--a value of about 14.8 to 1 by volume. However, the potentiometric sensor is not very sensitive to changes in oxygen partial pressure away from this point because of the logarithmic dependence of the EMF on the oxygen partial pressure.
It is advantageous to operate gasoline power piston engines with excess oxygen to improve fuel economy and reduce hydrocarbon emissions. To maintain stable combustion away from stoichiometry and enable engines to operate in the excess oxygen (lean burn) region, several limiting-current amperometric sensors have been reported. These sensors typically show reproducible limiting current plateaus with an applied voltage caused by gas diffusion overpotential at the cathode. The sensor current plateau is generally linearly proportional to the concentration of oxygen in the external environment and the oxygen concentration. These characteristics are obtained by limiting the diffusion of oxygen through a gas diffusion barrier.
Two types of gas diffusion barriers are currently being evaluated: (1) a cavity with a small diffusion hole; and (2) a porous ceramic layer on the cathode to limit the oxygen transfer rate from the ambient gas. The aperture-type is relatively difficult to manufacture and requires that the aperture remain unplugged. The porous-type is easy to manufacture, but control of the porosity is difficult and the ceramic may provide a changing pore morphology over time.
W. Weppner, "Tetragonal Zirconia Polycrystals--A High Performance Solid Oxygen Ion Conductor," Solid State Ionics 52, 15-21 (1992), suggests that a solid mixed oxygen ion and electronic conductor might be tried to replace an aperture, where the material has a suitable diffusion constant for oxygen. However, there is no teaching about acceptable materials and design parameters for use with solid state electrolytes, e.g., tetragonal zirconia polycrystals (TZP) or cubic stabilized zirconia (CSZ), which are both forms of yttria-doped zirconia.
In accordance with the present invention, suitable solid mixed oxygen ion and electronic conductors are provided and operating conditions are established.
Accordingly, it is an object of the present invention to provide materials as solid mixed oxygen ion and electronic conductors for use in a solid-state oxygen sensor.
Another object of the present invention is to determine suitable operating parameters for oxygen sensors with solid oxygen ion and electronic conductors to provide suitable sensitivity.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.